Universal Rendering Pipeline

A computer game doesn’t display finished images; it recalculates them anew at every moment. The computer only knows where objects are positioned in space, what colors they have, and where lights are placed. From that, it has to produce a flat image for the monitor sixty times per second. This process is called rendering, and the fixed sequence behind it is called a rendering pipeline. The Universal Rendering Pipeline, or URP for short, is such a sequence from the widely used development environment Unity, with which many games and apps are built. Its goal is to deliver usable images equally well on weak and powerful hardware.

One pipeline for phone and console alike

In the past, developers had to adapt their game separately for every device category. A phone has a tiny graphics chip and a battery, while a game console has plenty of computing power and a power outlet. Anyone who wanted to serve both essentially had to build the graphics twice. That cost months of work time and produced two versions that had to be maintained separately.

The URP solves this problem by having the same pipeline expend more or less effort depending on the device. On the phone, shadows are calculated more coarsely and fewer lights are allowed, while on the PC more are permitted. The scene, the materials, and the code remain the same. That is the reason for the word “universal” in the name.

Economically, that is the actual point. Small studios with few people can use it to release a game simultaneously for Android, iOS, PC, and console. This is one of the reasons Unity is so widespread among indie studios and in the mobile sector. For industries outside of games, such as architecture or automotive design, the same advantage applies to 3D visualizations.

From triangle to finished frame

Every 3D object consists of many small triangles, the so-called mesh. The pipeline first checks which objects are visible at all. Anything that lies behind the camera or is hidden by a wall is immediately discarded. This sorting-out step often saves more computing time than any other step.

Afterward, the triangles are converted into pixels. For each point, a small program runs on the graphics card, a shader, which determines the final color. It takes into account the object’s texture, the angle to the light source, and whether the point lies in shadow. At the end, effects such as blur or color correction are applied over the entire image.

The technical trick behind URP is called single-pass forward rendering. Put simply, each object is computed only once as far as possible, including all the lights falling on it. The competing method, deferred rendering, breaks the image down into several intermediate layers and can thereby handle a very large number of lights. However, it requires memory bandwidth that phones don’t have.

URP, HDRP, and the dispute over Unity’s fees

Besides the URP, Unity also offers the High Definition Rendering Pipeline, or HDRP for short. It targets expensive hardware and delivers more realistic reflections and lighting moods. Anyone who launches a game and finds only a few, but effective, sliders in the graphics options is often looking at a URP title. A common misconception is that the URP is the “inferior” variant. It is simply built toward a different goal, namely stable frame rates on a broad range of hardware.

In the news, the term mostly comes up in connection with Unity itself. When the company announced a fee per game installation in 2023, developers publicly discussed switching to the competing Unreal Engine or to the free Godot. Such a switch means redoing the entire graphics setup from scratch, because shaders and pipelines cannot be transferred. How firmly a studio is tied to a pipeline is therefore also a question involving millions of dollars.